The present invention is directed to a golf car having an electric propulsion system and, more particularly, to a golf car incorporating a shunt wound separately excited direct current electric motor propulsion system.
Electrically propelled vehicles are in use in numerous applications. For example, fixed rail transit vehicles and locomotives are typically propelled by electric motors as are various types of off-highway vehicles in the mining industry. Electric power for these types of vehicles is provided by external sources or by on-board engines and generator sets. Thus, the range of velocity and horsepower developed by these vehicles is controlled to a large extent by a virtually unlimited power source which permits use of large series wound direct current (DC) motors to provide tractive effort.
In some classes of vehicles, such as, for example, golf cars, the power source is limited to batteries because the vehicle must be kept relatively small while permitting maximum mobility. Historically, it has been the general practice to equip electrically powered golf cars with series wound DC electric motors. While such series wound motors provide satisfactory operating characteristics on generally flat golf courses, any course which requires that the vehicle be driven over hilly terrain has typically relied on internal combustion engine powered golf cars since the characteristics of the series wound DC motor made the electrically propelled golf cars undesirable on such courses. Referring briefly to FIG. 1, there is shown a typical characteristic curve for a series wound DC motor which shows that the motor produces its maximum torque at very low speeds but that torque is inversely proportional to vehicle speed. Accordingly, as a golf car attempts to climb a relatively steep grade, speed rapidly falls off as the required torque increases. Conventional golf cars using series wound DC motors will typically stall on a hill grade of about 40%.
It is known that shunt wound DC electric motors using separately excited armature winding and field winding controls can provide motor operating characteristics that have certain advantages over the series wound DC motor. In particular, the point at which speed begins to decrease in response to increased torque commands can be shifted considerably on the speed torque curve so as to improve the performance of a vehicle equipped with a shunt wound separately excited motor. However, controls for such motors have typically been too expensive to use on low end applications such as golf cars. In addition, the shunt wound motor has had other concerns such as the inability to provide retarding torque as the vehicle is driven down a hill. In particular, if the shunt wound motor is applied to a golf car and is proceeding down a hill of about 30 to 40% grade, the velocity of the vehicle can exceed the mechanical maximum velocity of the shunt wound motor causing the motor to mechanically fail and disable the vehicle. Furthermore, such high speeds in a golf car can be dangerous for passengers of that car. Still another disadvantage historically associated with the use of shunt wound motors in golf cars is the possibility of a runaway unattended golf car. For example, if the golf car is parked on or near the crest of a hill without setting the hill brake, the golf car could begin to roll after the passengers have left the car and accelerate to speeds which could destroy the mechanical drive system of the car or damage anything in the path of the golf car.